1. Field of the Invention
The invention relates to a waveform equalizer and, more particularly, to a waveform equalizer for performing a waveform equalization to a read signal read out from a recording medium. The invention also relates to a data reproducing apparatus using the waveform equalizer.
2. Description of Related Art
Optical disc record information reproducing apparatuses are widely used as one type of recording medium reading systems. A known technique in this field is that a photoelectric conversion output of reflection light from an optical disc which is derived by irradiating read light to the optical disc is obtained as an RF signal or a read signal, a waveform of the read signal is corrected by a waveform equalizer, a waveform interference is reduced, and thereafter, a binarizing process is performed, thereby generating a digital read signal.
In this technique, asymmetry of the read signal can be corrected at the time of the binarizing process.
More specifically, there are a variation in power of the recording light which is used for mastering at the time of the recording information to an optical disc and a variation in a length of pit as a recording mark which is actually formed on the optical disc by the recording light. When the recording optical power is not proper, there generally occurs a phenomenon such that each pit which is formed becomes longer or shorter little by little by the same length than the standard length at the positions before and after the length direction. That is, the symmetry between the pit and the nonpit portion cannot be maintained. Such a phenomenon is termed asymmetry.
FIG. 1 shows a state where a read signal obtained by reading a recording signal without asymmetry is binarized on the basis of a predetermined threshold value (in this example, zero). In this case, proper binary read signals having a duty ratio 50% corresponding to the recording signal are obtained in both of a high level portion and a low level portion.
FIG. 2 shows a case that a recording signal with asymmetry is read. Since the read signal in which the threshold value is shifted from a proper value is obtained, if the signal is binarized by the same threshold value as that in FIG. 1, the duty radio becomes shifted and an improper binary read signal is produced in which the high level portion and the low level portion do not correspond to the recording signal.
The asymmetry cannot be completely suppressed in a mass producing step of optical discs. The asymmetry of the read signal itself in a disc reading system also fluctuates depending on the wavelength of a read laser beam of an optical pickup being used.
A disc of a CD (compact disc) system has been standardized so that asymmetry lies within a certain range. In the reading system, the asymmetry is automatically corrected by using the nature of a read signal which is presumed in accordance with the CD system. In more detail, there used is a method of tracking-detecting the center of an eye pattern of a read signal and binarizing the read signal by using the detected center level as a threshold value by using a nature such that the DC component of the EFM (Eight to Fourteen Modulation) signal itself which is recorded to the CD is equal to zero, a length of continuation (what is called a run length) of logical value xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d of the data by the EFM is limited to a length within a range of 3T to 11T (1T is equal to a period of one channel bit), and the like.
Hitherto, it is considered that a tracking ability of the eye pattern in the above method is sufficient for the disc of the CD system.
However, in the case of discs having a capacity larger than that of the CD and the recording of a higher density is performed such as a DVD (Digital Video disc or Digital Versatile disc) or the like which are progressively being put into practical use, a pit length corresponding to each run length of the recording data is fairly shorter than that of the CD and there is a case where the eye pattern tracking ability by the above method is insufficient.
Explanation will now be made with respect to the 8/16 modulation of the DVD as an example. In the case of recording by setting the pit length to a short length, if a recording power is deviated from the optimum value, the center of the eye pattern of a signal of 3T as a minimum run length in the read signal is deviated from the center of the eye pattern of a signal of 4T or more, so that the read signal deteriorates. The eye pattern center corresponds to a threshold value such that a pulse width of the signal obtained by binarizing the read signal is equal to a pulse width of the recording signal.
A change of the center of the eye pattern for the recording power is now schematically shown in FIG. 3. Since the signal of 3T is near the limit of a resolution of an LBR (laser beam recorder) as a signal recording system to the optical disc, the center position of the eye pattern of the signal of 3T changes sensitively for the recording power. That is, the signal of 3T has a characteristic of a large inclination as shown in the figure.
On the other hand, since the signal of 4T or more has a surplus in the resolution of the LBR, a change of the center of the eye pattern of the signal of 4T or more for the recording power is gentle.
When the recording power is smaller than the optimum value, therefore, the center of the eye pattern of the signal of 3T is brighter than the center of the eye pattern of the signal of 4T or more (near the mirror level). That is, as shown in FIG. 4A, when the recording power is too small, a proper threshold value of the signal of 3T becomes higher than that of the signal of 4T or more, for the time of binarization of the read signal. When the recording power is larger than the optimum value, the center of the eye pattern of the signal of 3T is darker than the center of the eye pattern of the signal of 4T or more. That is, as shown in FIG. 4B, at the time of a too large recording power, upon binarization of the read signal, a proper threshold value of the signal of 3T is lower than that of the signal of 4T or more.
When the signal of 3T and the signal of 4T or more are binarized by the same threshold value, therefore, the deviation of the center of the eye pattern of the signal of 3T, namely, becomes the deviation of the duty due to the asymmetry, thereby causing a jitter to occur in the read signal.
As mentioned above, the center of the eye pattern of the signal of 3T mainly representing the shortest pit sensitively changes for the recording power due to the lack of resolution of the LBR, so that a margin of the recording power is narrowed. A fact that the signal indicative of the shortest pit exerts a large influence on the quality of the read signal to an extent which cannot be ignored becomes a problem which was not found in the CD or the like.
In the case of recording media of the WORM type and the rewritable type, since the recording medium itself which is used and a recording apparatus, and further, a recording environment depend on each user, it is presumed that asymmetry occurs due to a variation of the recording power. It is extremely important to cope with the asymmetry in order to maintain high playability on products.
The invention is made in consideration of the foregoing points and it is an object of the invention to provide a waveform equalizer which can improve a quality of a read signal by certainly correcting asymmetry in the read signal and to provide a data reproducing apparatus using the waveform equalizer.
According to the invention, there is provided a waveform equalizer for performing a waveform equalization to a read signal, comprising: inclination discriminating means for discriminating an ascending slope portion and a descending slope portion of the read signal; a filter for performing a filtering process to the read signal; and characteristics switching means for switching frequency characteristics of the filter in accordance with a discrimination output of the inclination discriminating means.
In the waveform equalizer with the above construction, the filter has a first filter in which frequency characteristics for the ascending slope portion are set and a second filter in which frequency characteristics for the descending slope portion are set, and the characteristics switching means can select an output signal of either one of the first and second filters and use the selected output signal as an output signal of the waveform equalizer.
The filter is an FIR filter and the characteristics switching means can switch the frequency characteristics by changing values of tap coefficients of the FIR filter.
Further, the frequency characteristics under the discrimination of the ascending slope portion can be set in accordance with a sampling value near a zero-cross point in the ascending slope portion of the corresponding read signal and the frequency characteristics under the discrimination of the descending slope portion can be set in accordance with a sampling value near a zero-cross point in the descending slope portion of the corresponding read signal.
According to the invention, there is provided a data reproducing apparatus for obtaining reproduction data by binarizing a read signal read out from a recording medium, comprising: sampling means for forming a sampling value series read signal by sampling the read signal; a waveform equalizer including inclination discriminating means for discriminating an ascending slope portion and a descending slope portion of the sampling value series read signal, a filter for performing a filtering process to the sampling value series read signal, and characteristics switching means for switching frequency characteristics of the filter in accordance with a discrimination output of the inclination discriminating means; and binarizing means for forming the reproduction data by binarizing an output signal of the waveform equalizer.